Sensory System

Sensory Receptors in the Skin

Mechanoreceptors

Four main types of mechanoreceptors detect different tactile stimuli:

• Meissner’s Corpuscles: Rapidly adapting, located in glabrous (hairless) skin; detect light touch; most dense in fingertips and lips
• Pacinian Corpuscles: Rapidly adapting, detect high-frequency vibration; responsible for vibratory sensation tested with 128 Hz tuning fork
• Merkel’s Discs: Slowly adapting, detect pressure, fine details, edges, and textures; highest density in fingertips; critical for two-point discrimination
• Ruffini Endings: Slowly adapting, detect skin stretch and sustained pressure; contribute to proprioception and joint position sense

Thermoreceptors

• Cold Receptors: Free nerve endings with A-delta fibers
• Warm Receptors: Free nerve endings with C fibers

Nociceptors (Pain Receptors)

• A-delta Nociceptors: Myelinated fibers; mediate sharp, well-localized “first pain”; activated by mechanical and thermal stimuli
• C Fiber Nociceptors: Unmyelinated; mediate dull, poorly localized “second pain”; most numerous type
• Silent Nociceptors: Normally inactive but become responsive after tissue injury; contribute to inflammatory hyperalgesia

Ascending Sensory Pathways

Dorsal Column-Medial Lemniscal System

Modalities: Fine touch, vibration, proprioception, two-point discrimination

Pathway:

1. First-Order Neurons: Cell bodies in dorsal root ganglia (DRG); large diameter, heavily myelinated A-beta fibers
2. Spinal Cord Entry: Enter via medial division of dorsal root; ascend ipsilaterally in dorsal columns
   • Gracile fasciculus (medial): Carries information from lower limbs and lower trunk (T6 and below)
   • Cuneate fasciculus (lateral): Carries information from upper limbs and upper trunk (above T6)
3. First Synapse: Nucleus gracilis and nucleus cuneatus in caudal medulla
4. Decussation: Second-order neurons cross as internal arcuate fibers at level of medulla; form medial lemniscus
5. Medial Lemniscus: Ascends contralaterally through medulla, pons, and midbrain; maintains somatotopic organization (cervical lateral, sacral medial)
6. Second Synapse: Ventral posterolateral (VPL) nucleus of thalamus
7. Third-Order Neurons: Project via posterior limb of internal capsule to primary somatosensory cortex (S1) in postcentral gyrus

Spinothalamic Tract

Modalities: Pain and temperature (lateral), crude touch and pressure (anterior)

Pathway:

1. First-Order Neurons: Cell bodies in DRG; small diameter A-delta (pain/temperature) and C fibers (pain/temperature/crude touch)
2. Spinal Cord Entry: Enter via lateral division of dorsal root; ascend or descend 1-2 segments in Lissauer’s tract before synapsing
3. First Synapse: Substantia gelatinosa (lamina II) and nucleus proprius (laminae III-IV) in dorsal horn
4. Decussation: Second-order neurons cross via anterior white commissure within 1-2 segments of entry level
5. Lateral Spinothalamic Tract: Ascends contralaterally in anterolateral quadrant; pain and temperature
   • Somatotopic organization: Sacral lateral, cervical medial
   • New fibers added medially as tract ascends
6. Anterior Spinothalamic Tract: Ascends contralaterally just anterior to lateral tract; crude touch and pressure
7. Second Synapse: VPL nucleus of thalamus (also some fibers to intralaminar nuclei and reticular formation)
8. Third-Order Neurons: Project to primary somatosensory cortex (S1)

Spinocerebellar Tracts

Modalities: Unconscious proprioception to cerebellum for motor coordination

• Dorsal Spinocerebellar Tract: Ipsilateral pathway; Clarke’s column (C8-L2) to inferior cerebellar peduncle; carries proprioceptive information from lower limbs and trunk
• Ventral Spinocerebellar Tract: Crosses twice (so functionally ipsilateral); spinal border cells to superior cerebellar peduncle; monitors motor commands to lower limbs
• Cuneocerebellar Tract: Equivalent of dorsal spinocerebellar for upper limb; lateral (accessory) cuneate nucleus to inferior cerebellar peduncle

Central Relay Stations

Thalamus – The Sensory Gateway

The thalamus is the major relay station for all sensory information (except olfaction) en route to the cerebral cortex.

Ventral Posterior Nucleus

• VPL (Ventral Posterolateral): Receives body sensation (dorsal columns via medial lemniscus, spinothalamic tract)
  • Somatotopic organization: Leg lateral, arm medial
  • Projects to postcentral gyrus (S1)
• VPM (Ventral Posteromedial): Receives facial sensation (trigeminal pathways)
  • Receives from ventral trigeminothalamic tract (pain/temperature) and dorsal trigeminothalamic tract (touch/proprioception)
  • Projects to lower postcentral gyrus (S1)

Other Sensory Nuclei

• Medial Geniculate Nucleus (MGN): Auditory relay; receives from inferior colliculus; projects to primary auditory cortex (Heschl’s gyrus)
• Lateral Geniculate Nucleus (LGN): Visual relay; receives from optic tract; projects to primary visual cortex (V1, calcarine cortex)
• Intralaminar Nuclei: Pain modulation, arousal; receive spinothalamic and spinoreticular inputs; project diffusely to cortex

Somatosensory Cortex

Primary Somatosensory Cortex (S1)

Location: Postcentral gyrus (areas 3a, 3b, 1, 2)

Organization:

• Somatotopic representation (sensory homunculus): Proportional to receptor density, not body part size
• Inverted arrangement: Leg/foot at top (medial), hand in middle, face at bottom (lateral)
• Area 3b: Primary tactile processing; receives most direct thalamic input
• Areas 1 and 2: Texture and shape discrimination
• Area 3a: Proprioception

Secondary Somatosensory Cortex (S2)

Location: Parietal operculum (superior lip of Sylvian fissure)

Function:

• Bilateral representation of body
• Higher-order tactile processing
• Pain perception and integration
• Tactile memory and recognition

Posterior Parietal Cortex

Location: Superior parietal lobule (areas 5, 7)

Function:

• Multimodal sensory integration
• Spatial awareness and attention
• Sensorimotor transformation for reaching
• Lesions cause contralateral neglect, apraxia, astereognosis

Clinical Syndromes

Spinal Cord Lesions

Brown-Séquard Syndrome (Hemisection)

• Ipsilateral: Loss of vibration, proprioception, and fine touch (dorsal columns) below lesion; spastic weakness (corticospinal) below lesion
• Contralateral: Loss of pain and temperature (spinothalamic) 1-2 segments below lesion
• At Level: Ipsilateral dermatomal pain/temperature loss; LMN weakness in that myotome

Central Cord Syndrome

• Hyperextension injury or syringomyelia
• Suspended sensory level: Loss of pain/temperature in “cape” distribution across shoulders
• Arms more affected than legs (central spinothalamic fibers affected first)
• Preserved dorsal column function

Anterior Spinal Artery Syndrome

• Infarction of anterior 2/3 of spinal cord
• Bilateral loss of pain and temperature (spinothalamic)
• Bilateral weakness (corticospinal)
• Preserved vibration and proprioception (dorsal columns spared)

Tabes Dorsalis (Neurosyphilis)

• Degeneration of dorsal columns and dorsal roots
• Loss of proprioception and vibration
• Sensory ataxia with positive Romberg sign
• Lightning pains, Argyll Robertson pupils

Subacute Combined Degeneration (B12 Deficiency)

• Dorsal column and lateral corticospinal tract degeneration
• Loss of vibration and proprioception with spastic weakness
• Sensory ataxia with hyperreflexia
• Associated with megaloblastic anemia, glossitis

Brainstem Lesions

Lateral Medullary Syndrome (Wallenberg)

• Ipsilateral: Facial pain/temperature loss (spinal trigeminal), Horner’s, ataxia (inferior cerebellar peduncle), dysphagia (nucleus ambiguus)
• Contralateral: Body pain/temperature loss (spinothalamic tract)
• Key: Dissociated sensory loss (ipsilateral face, contralateral body)

Medial Medullary Syndrome

• Ipsilateral: Tongue weakness (hypoglossal nucleus)
• Contralateral: Hemiparesis (pyramid), loss of vibration/proprioception (medial lemniscus)

Thalamic Lesions

Thalamic Pain Syndrome (Dejerine-Roussy)

• VPL/VPM infarction or hemorrhage
• Contralateral hemibody sensory loss (all modalities)
• Severe, intractable burning pain (central post-stroke pain)
• Hyperesthesia and allodynia develop weeks to months later
• Often associated with hemiataxia (if ventral lateral nucleus involved)

Cortical Lesions

Parietal Cortex Lesions

• Primary Sensory Cortex (S1): Contralateral hemianesthesia (all modalities); astereognosis (inability to recognize objects by touch)
• Secondary Sensory Cortex (S2): Bilateral tactile deficits; tactile agnosia
• Posterior Parietal Cortex: Contralateral neglect (especially right hemisphere), apraxia, agraphesthesia (inability to recognize numbers written on skin)

Clinical Testing

Bedside Examination

Modality	Pathway Tested	Clinical Test
Light Touch	Dorsal columns (primarily)	Cotton wisp or finger; compare side to side
Pinprick	Spinothalamic (lateral)	Safety pin; sharp vs dull; map sensory level
Temperature	Spinothalamic (lateral)	Tuning fork or cold metal; compare side to side
Vibration	Dorsal columns	128 Hz tuning fork on bony prominences; distal to proximal
Proprioception	Dorsal columns	Move toe/finger up or down with eyes closed
Two-Point	Dorsal columns + cortex	Calipers; normal fingertip 2-3mm; dorsum hand 20-30mm
Stereognosis	Dorsal columns + parietal cortex	Identify object in hand with eyes closed (coin, key, etc.)
Graphesthesia	Dorsal columns + parietal cortex	Identify number written on palm with eyes closed
Romberg Test	Dorsal columns or peripheral nerves	Stand with feet together, eyes closed; positive if falls

Interpretation Tips

• Dermatomal Pattern: Radiculopathy; follows specific dermatome with motor weakness in corresponding myotome
• Peripheral Nerve Pattern: Specific nerve distribution; may have motor and autonomic findings
• Stocking-Glove Pattern: Length-dependent peripheral neuropathy; distal symmetric sensory loss
• Hemisensory Loss: Contralateral thalamic or cortical lesion; all modalities equally affected
• Sensory Level: Spinal cord lesion; determine exact level by pinprick exam ascending from feet

Summary

Understanding somatosensory pathways requires knowing:

1. Receptor types and their specific functions
2. Pathway anatomy including where each pathway decussates
3. Somatotopic organization at each level from spinal cord to cortex
4. Clinical localization based on pattern of sensory loss

The key to localization is determining whether sensory loss is dissociated (different modalities affected differently), unilateral or bilateral, and whether it follows a dermatomal, peripheral nerve, or central pattern. This systematic approach allows precise anatomical localization essential for board examinations and clinical practice.